The present invention relates to rechargeable, polymer lithium or lithium ion batteries and more particularly to a separator component with improvements in function and manufacturability.
In the construction of any battery, six elements must be present, namely, a positive and negative electrode, a housing, a separator, an electrolyte, and current collectors. The separator is an important element because it must be permeable to ions moving back and forth during charging and discharging, but must not permit flow of electrons directly from pole to pole, which shorts out the current, and prevents electrons from flowing from anode to cathode through a circuit. In rechargeable lithium ion batteries, which are generally formed of very thin layers of the components, it is important that the distance between electrodes be as short as possible to permit efficient ion transfer, but not so short as to permit electron flow.
Separators have been constructed of very thin sheets of plastic, which are rendered porous by removal of plasticizers from cast films. In general a nonaqueous environment is maintained, since lithium salts are notoriously reactive with electrode components in aqueous solutions. Aprotic organic solvents such as propylene carbonate or ethylene carbonate are commonly used in which lithium salt solutes are readily dispersable. Other solvents are tetrahydrofuran, 1,2-dimethoxyethane, dimethyl carbonate and diethoxyethane. For a discussion of conventional solvent/lithium solute systems, see S. Hossain, xe2x80x9cRechargeable Lithium Batteries (Ambient Temperature)xe2x80x9d, in Handbook of Batteries and Fuel Cells, D. Linden, Ed., McGraw-Hill, 2nd Ed., 1995. The plastic separator, of course, must be stable to the solvent selected.
U.S. Pat. Nos. 4,138,459, 3,801,404, and 3,843,761 disclose a method of producing a porous plastic separator by stretching a crystalline polyolefin film at a temperature below its transition temperature. Performance of such stretched polymers is impaired, however, because lack of control over uniform pore size leads to excessive distance between electrodes. U.S. Pat. No. 4,994,335 discloses a stretching process in which the strain rate and temperature of the process are tightly controlled, to produce microporosity in only one direction, so that fine fibrils are connected between adjacent unstretched planar flat portions in two dimensions.
A serious problem arises when the electrolyte lithium salts react in the complete cell to form dendrites of lithium, which tend to short out the battery by filling the void spaces of the separator and creating a conductive pathway. U.S. Pat. No. 5,427,872 discloses a method of preventing dendrite shorting, by disposing a dendrite reactive inert polypropylene or polyethylene composite separator along with a fluorinated polymer such as polyfluoroethylene. Porosity is maintained by prevention of dendrite penetration of the electrode-protective second separator.
In an alternative approach, U.S. Pat. Nos. 5,460,904, 5,296,318, 5,429,891 disclose a separator comprising a self-supporting film of a copolymer of vinylidene fluoride (PVdF) and hexafluoropropylene (HFP). Prior to casting, the copolymers are mixed with a medium to high temperature boiling plasticizer solvent such as dibutyl phthalate (DBP) and a filler such as SiO2. After casting, the plasticizer is leached out (extracted) with a solvent such as ethylether, and replaced by cell electrolyte. The spaces occupied by the plasticizer are filled with electrolyte in a communicating network of vacuoles permitting ion diffusion. In a preferred embodiment, the xe2x80x9cdryxe2x80x9d separator may be joined with the electrodes by lamination under heat and pressure prior to electrolyte loading.
U.S. Pat. No. 4,550,064 discloses a separator comprising two layer, the first inner layer composed of microporous polypropylene (Celgard) or fiberglass whose surfaces are made more hydrophilic by coating with imidazoline. These separators are to be used in combination with a positive electrode manufactured with a propylene/ethylene elastomer binder.
The foregoing separators have certain disadvantages. Multi-layer separators have a step gradient of porosity with different diffusion constants for electrolyte in each layer. Efficiency of ion transfer may be impaired which interferes with the discharge rate of the battery, and may adversely affect capacity. In the case of the separators made from PVdF and HFP, solvent leaching causes brittleness which leads to a significant level of product failure during lamination. Co-mingling of leaching solvent and plasticizer which cannot be reused also creates a disposal problem.
In accordance with the present invention, a separator suitable for use in a secondary lithium battery is made of a pre-formed porous non-woven mat comprising a first homopolymer of polypropylene, polyethylene, or polyvinylalcohol, coated with a second homopolymer. Porosity of the homopolymeric coating, which may preferably be polyvinylidene difluoride, is obtained by first mixing the homopolymer with a low boiling solvent selected from one or more non-aromatic aliphatic diesters, followed by forming a layer of the polymer diester mixture. The resultant separator, positionable between anode and cathode electrodes, comprises a porous core layer matrix, the layer having opposite surfaces, and at least one of the surfaces having a porous homopolymeric coating applied thereto. Then remove the solvent plasticizer by applying a vacuum. The separator construct is sufficiently permeable to electrolyte ions to minimally discharge a secondary battery having a carbonaceous or other lithium intercalation anode and a lithium metal oxide cathode, 30 percent at 2 C.
In use, the separator is a component of a rechargeable lithium battery comprising a housing, preferably sealed in plastic, electrodes contained in the housing including a carbonaceous or other lithium intercalation anode consisting of amorphous graphite, coke, filamentous carbon, or combinations thereof, and a lithium metal oxide cathode, an electrolyte solution in the housing, the electrolyte solution containing a lithium metal salt capable of ionizing in an organic solvent, the salt being dispersed in the organic solvent, current collectors electrically connected and disposed in contact with the electrodes, and the separator disposed between the electrodes, the separator including a fibrous matrix core having surfaces, and a porous hombpolymeric coating layer applied to at least one of the matrix core surfaces.
In the process of manufacturing the separator, a fibrous polymeric core matrix is coated with a second polymer mixture containing a non-aromatic solvent plasticizer of low boiling temperature, and acetone, and then removing the plasticizer by heating under vacuum, to create a microporous layer conductive to lithium salt ions. A rechargeable battery of simple unitary construction is produced by aligning in stacked registration a carbonaceous or other lithium intercalation anode, a lithium metal oxide cathode, and the separator, the separator being disposed between the carbonaceous or other lithium intercalation anode and entrained lithium salt polymeric cathode, and laminating the anode, cathode, and separator by application of heat and pressure sufficient to cause adhesion of the anode, cathode, and separator into a unitary structure.
In a further battery embodiment, an additional cathode is added to the laminated structure of the secondary lithium ion battery by positioning a second separator on the outer side of the anode and interfacing in aligned stacked registration a second cathode layer, to form a binary structure represented by the structure: +sxe2x88x92s++sxe2x88x92s+ wherein + is a cathode layer, xe2x88x92 is a anode layer, and s is a separator. The thickness of the layers is adjusted to balance the specific charge capacity of anode and cathode.